FIG. 1 shows a scheme for testing a radar. In this scheme, a radar target simulator system 10 is located a distance r from a radar under test (RUT)12, and is used to generate an apparent target 14 that is located a greater distance R from radar 12. The characteristics of the simulated target, such as delay and radar cross section (RCS), are established by the simulator so that the performance and functionality of the RUT can be verified. Radar target simulators can be designed for a specific radar to be tested or can be designed for use with the testing of many different types of radars. Examples of the latter of these can be found in U.S. Pat. No. 5,223,840 and in the article titled Universal Radar Moving Target Transponder, presented at the IEEE 1996 National Radar Conference, Ann Arbor, Mich., May 13-16, 1996. Both of these publications are incorporated by reference herein.
To verify that the RUT is functioning properly, it is necessary that the target simulator equipment used to test it be accurately calibrated. Calibrating a radar target simulator requires knowing the gain and delay through the simulator, including all of its components such as the radar target generator and its associated antennas and transmission cables.
In many radar test configurations, the simulator will use antennas that are located a significant distance from its accompanying radar target generator 16. These antennas, shown as elements 18 and 20 in FIG. 1, and the associated cables, elements 24 and 26, may even be in locations that are difficult to physically access, such as on towers or rooftops.
Traditionally, calibration of the simulator for gain and delay are performed in two distinct operations.
While it is easy to measure the gain of the radar target generator itself (from its input to output), ascertaining the gain associated with the transmission cables running from the target generator to its antennas can be problematic. This is particularly true when sections of the receive and transmit cables running to and from the antennas are changed over the course of time, requiring ideally that separate gain measurements be made for the target generator and for the cables. An even greater challenge is measurement of the receive and transmit antennas gains themselves. Vendor-provided specifications of antenna gain are frequently subject to significant errors. Though archived measurements can be useful, they often must be fitted/interpolated for the particular frequency of each RUT. Additionally, archived values can change over time as an antenna degrades. Thus the over-all determination of the parameters which enter into the gain calibration of a radar target simulator is a fragmented process, not integrated into a single measurement.
Determining the over-all delay through a radar target simulator is also desirable in order to properly calibrate the simulator. Traditional measurements of over-all delay are subject to some of the same problems as gain measurement such as the inaccessibility of cables and use of a fragmented measurement process.
An apparatus and process makes it possible to quickly and accurately determine an equivalent for the RCS and range delay of a signal provided by a radar target simulator system. This calibration process considers the gain and delays associated with a radar target generator, the receive and transmit antennas associated with the generator, and the receive and transmit cables going to and from the generator to the receive and transmit antennas, respectively. Accurately determining gain and delay through the radar target simulator system permits the simulator to provide a well-calibrated RCS and range for radars it is testing and calibrating.
An object of this invention is to provide a calibrator for a radar target simulator that permits the over-all gain of the simulator to be determined.
Another object of this invention is to provide a calibrator for a radar target simulator that permits the RCS of a simulator signal as received by a RUT to be determined.
A further object of this invention is to provide a calibrator for a radar target simulator that permits the over-all delay through the simulator to be determined.